This invention relates to an apparatus and a method for refining alkaline solution such as, for example, sodium hydroxide solution, potassium hydroxide solution.
Alkali chemicals have been used in steps of polishing and cleaning a wafer during a fabricating process of a silicon wafer which is a semiconductor base, and as today's industry has been highly and finely developed, NaOH solution of extremely high-purity and high concentration has been required, whose concentration is for example about 10 to 50 wt % and whose impurity concentration is for example equal to or less than about 10 ppb when sodium hydroxide solution (NaOH solution) is used as alkali chemicals.
As a conventional manufacturing method of NaOH solution, such a method is known that salt solution is fed to an anodic chamber of an electrolytic bath which is divided into the anodic chamber and a cathodic chamber by a cation-exchange membrane, and that a sodium ion passes through the cation-exchange membrane from the anodic chamber side to the cathodic chamber to proceed a generating reaction of NaOH solution in the cathodic chamber. The concentration of the NaOH solution obtained as above is at most 30 to 35 wt %, and when trying to make a solution of high concentration from this, a concentrating can for example has been used to concentrate the solution, while such a method needs a large equipment and a long processing time.
Therefore, the present inventors have studied techniques where an electrolytic bath 1 is divided into an anodic chamber 12 and a cathodic chamber 13 by a cation-exchange membrane 11 as shown in FIG. 3 for example, and a base NaOH solution of high impurity concentration is supplied to the anodic chamber 12 to perform electrolysis, whereby a refined NaOH solution of lower impurity concentration and higher concentration than the base NaOH solution is obtained in the cathodic chamber 13. In this method, a sodium ion (Na+) generated in the anodic chamber 12 passes through the cation-exchange membrane 11 to the cathodic chamber 13, and whereby sodium hydroxide which is a hydroxide of sodium is generated in the cathodic chamber 13 so as to generate a sodium hydroxide solution by dissolving this sodium hydroxide in water.
There exists metal as impurities in the anodic chamber 12 at this time, but since this metal exists as an anion or precipitates as a hydroxide in alkaline atmosphere, the metal cannot pass through the cation-exchange membrane 11. Therefore, since the impurities do not get into the cathodic chamber 13, the obtained sodium hydroxide solution will be of an extremely low impurity concentration, and since Na+ migrates to the cathodic chamber 13 so as to gradually increase the concentration of the NaOH solution in the cathodic chamber 13, the refined NaOH solution will be of higher concentration than the base NaOH solution.
By the way, when electrolysis is performed at a certain electric current density in the method described above, only a certain amount of ions migrates from the anodic chamber 12 to the cathodic chamber 13 through the cation-exchange membrane 11. However, it is already known that the number of H2O molecules with which NaOH is hydrated differs depending on the concentration, and whereby the number of H2O molecules with which Na+ migrates from the anodic chamber 12 differs depending on the concentration of the NaOH solution in the anodic chamber 12. Therefore, when the concentration of the base NaOH solution supplied to the anodic chamber 12 changes, the concentration of the refined NaOH solution in the cathodic chamber 13 also changes.
Here, although a certain amount of the base NaOH solution is to be supplied to the anodic chamber 12 using a metering pump, the concentration of the NaOH solution in the anodic chamber 12 is not always constant, so that there exists a problem that the concentration of the refined NaOH solution is not stable.